Add section about eager loading of files

[SimonSimCity]

Eager loading is - as far as I can see - only really explained in the release notes of Meteor 1.7 (https://docs.meteor.com/changelog.html#changes-21) but neither in the guide for application structure (https://guide.meteor.com/structure.html#load-order) nor in the guide for testing (https://guide.meteor.com/testing.html#test-modes).

Since it is the default for quite a while now (meteor/meteor#9715) this behavior should be documented in the Meteor Guide as well.

Links which might help describing the behavior are the PRs of both of those changes:

• Support a meteor.mainModule section in application package.json files. meteor#9690
• Support a meteor.testModule section in application package.json files. meteor#9714

I also tried to summarize it in a stackoverflow answer: https://stackoverflow.com/a/64116026/517914